LESSONS ON SITE LESSONS ON SITE CHARACTERIZATION CHARACTERIZATION

GLEANED FROM FORENSIC GLEANED FROM FORENSIC STUDIESSTUDIES

J. David RogersJ. David RogersDepartment of Geological Sciences & Department of Geological Sciences &

Engineering Engineering University of MissouriUniversity of Missouri--RollaRolla

KARL TERZAGHIKARL TERZAGHIAustrian engineer 1883Austrian engineer 1883--19631963Graduate training in Graduate training in geomorphologygeomorphologyWorld traveler and astute World traveler and astute observer of natureobserver of natureFathered soil mechanicsFathered soil mechanicsAwarded 4 Norman Medals Awarded 4 Norman Medals Promoted appreciation of Promoted appreciation of engineering geology by engineering geology by civil engineering professioncivil engineering profession

TERZAGHI’S METHODTERZAGHI’S METHODStudy geology and geomorphology of region Study geology and geomorphology of region surrounding project sitesurrounding project siteGather all forms of existing data, including Gather all forms of existing data, including geologic, soils, hydrologic, geologic, soils, hydrologic, meteorlogicmeteorlogicIdentify “missing gaps” in geo informationIdentify “missing gaps” in geo informationMake onMake on--thethe--ground reconnaissance of the ground reconnaissance of the site; note dominant site; note dominant erosionalerosional processesprocessesFormulate a working hypothesis regarding the Formulate a working hypothesis regarding the nature of likely subsurface conditionsnature of likely subsurface conditionsDevelop plan of site exploration, designed to Develop plan of site exploration, designed to fill missing gapsfill missing gaps

TERZAGHI’S REPORTSTERZAGHI’S REPORTSHis first report was a summary of site geology and what he perceived to be the data gaps

The second report summarized the site specific investigation and either characterized site conditions or demanded additional work

Terzaghi made thorough use of ongoing construction observations to verify

assumed site conditions

Always made provisions for changing plans whenever different conditions were observed

EXPERIENCEEXPERIENCEGeologists only recognize those

features with which they have accumulated substantive experience

Mis-characterization most often occurs when people making the examinations have little previous experience in the area

Geologists use different technical terms, based on their experience, e.g. listric faults versus landslide slip surfaces, or colluvium vs slide debris

THE COLLUVIUM CATCH ALLTHE COLLUVIUM CATCH ALLColluviumColluvium is loose, is loose, heterogeneous, and heterogeneous, and incoherent mass of incoherent mass of soil material and/or soil material and/or rock fragments rock fragments deposited by deposited by rainwashrainwash, sheet , sheet wash, or wash, or downslopedownslopecreepcreep

Block diagrams showing cycle of colluviual deposition and erosion

Technical terms Technical terms connote genesisconnote genesis

Slide debris is often Slide debris is often mistaken for mistaken for colluviumcolluviumbecause the geologist because the geologist doesn’t see a slip surfacedoesn’t see a slip surface

Loose soil debris on a grassy slope will be deposited without a shear surface

The Dec 31, 1933The Dec 31, 1933--Jan 1, 1934 MontroseJan 1, 1934 Montrose--LaCresentaLaCresenta debris flows debris flows damaged or destroyed 600 homes and killed 44. 600,000 cubic damaged or destroyed 600 homes and killed 44. 600,000 cubic yards of material was deposited on the fan in one evening.yards of material was deposited on the fan in one evening.

Many mapped alluvial fans (Many mapped alluvial fans (QalQal) are actually debris fans, ) are actually debris fans, constructed by countless series of coalescing debris constructed by countless series of coalescing debris flows. One trench can expose the character and genesis flows. One trench can expose the character and genesis of underlying material. If it is matrixof underlying material. If it is matrix--supported, it was supported, it was likely deposited in a debris flow.likely deposited in a debris flow.

MATRIX SUPORTEDMATRIX SUPORTEDMATERIALSMATERIALS

Debris flow deposits Debris flow deposits are typically are typically deposited on channel deposited on channel slopes around 10% slopes around 10% They are They are characterized by fine characterized by fine grained matrix grained matrix between between clastsclasts, large , large variation in variation in clastclast and and particle sizes, and particle sizes, and often, by inverse often, by inverse sorting sorting

SLOPE CREEPSLOPE CREEPGeologists are often Geologists are often asked to estimate asked to estimate depth and depth and magnitude of slope magnitude of slope creepcreepCreep is a function Creep is a function of soil plasticity, of soil plasticity, slope inclination, slope inclination, seasonal moisture seasonal moisture variation and height variation and height of slopeof slope

Slope creep can exert passive soil pressures against embedded foundations

AGING FACTORSAGING FACTORS

Clayey soils and argillaceous materials are susceptible Clayey soils and argillaceous materials are susceptible to slope creep. Be careful to advise clients of likely to slope creep. Be careful to advise clients of likely movements and need for ongoing maintenancemovements and need for ongoing maintenanceThese comparative views of the same highway cut were These comparative views of the same highway cut were taken in 1954 and 1986, 32 years apart. The benches taken in 1954 and 1986, 32 years apart. The benches were gone.were gone.

DIFFERENTIAL HEAVEDIFFERENTIAL HEAVEDifferential heave is a Differential heave is a recurring problem recurring problem whenever expansive whenever expansive clay clay shalesshales are are sandwiched between sandwiched between nonnon--expansive beds, expansive beds, as sketched at leftas sketched at leftExpansive soils are Expansive soils are the #2 cause of the #2 cause of property loss in the property loss in the United States, United States, second only to dry second only to dry rot rot

PAST LAND USEAGEPAST LAND USEAGEA preliminary check of A preliminary check of aerial photos is aerial photos is essential to understand essential to understand what a site was used what a site was used for prior to anticipated for prior to anticipated constructionconstructionThis shows the case of This shows the case of a subdivision in San a subdivision in San Jose, CA which was Jose, CA which was built over an old built over an old orchard orchard

FILLS AND FILLS AND OLD OLD

CHANNELSCHANNELS

Check old maps and air photos to see where channels Check old maps and air photos to see where channels and sloughs were located prior to infilling, draining or reand sloughs were located prior to infilling, draining or re--direction. This is a common problem in urban areasdirection. This is a common problem in urban areas

INTERPRETING SUBSURFACE DATAINTERPRETING SUBSURFACE DATASoils engineers tend to focus Soils engineers tend to focus on providing foundation on providing foundation recommendations, not on recommendations, not on detecting geologic contactsdetecting geologic contactsProper interpretation of Proper interpretation of subsurface information subsurface information requires experience on part requires experience on part of the drillers and the logging of the drillers and the logging geologistgeologistCirculation problems can Circulation problems can negate accurate assessments negate accurate assessments of cuttings depth and weak of cuttings depth and weak seams of horizons are easily seams of horizons are easily missed missed

CLASTS LARGER THAN SAMPLER SHOECLASTS LARGER THAN SAMPLER SHOE

Bedrock contacts are commonly misidentified when the sampler shoe encounters a floater of greater diameter, as sketched at lower left

MISINTERPRETATION MISINTERPRETATION OF DATAOF DATA

Subsurface data Subsurface data is easily is easily misinterpreted misinterpreted when working in when working in tectonically active tectonically active terrain.terrain.This shows before This shows before construction construction (upper) and after (upper) and after excavation (lower) excavation (lower) cross sections of cross sections of the same sitethe same site

Exploratory trenches can effectively expose geologic structures and the nature or genesis of key contacts

RECOGNIZING OLD RECOGNIZING OLD LANDSLIDESLANDSLIDES

Old landslide features can Old landslide features can be difficult to discern, be difficult to discern, even for experienced even for experienced engineering geologistsengineering geologistsOne needs to be One needs to be “looking” for such “looking” for such features in order to features in order to recognize themrecognize themCase at lower left was old Case at lower left was old slide reactivated on slip slide reactivated on slip plane inclined just 3 plane inclined just 3 degrees, without any degrees, without any visible moisture visible moisture

TOPOGRAPHICTOPOGRAPHICEXPRESSIONEXPRESSION

A key indicator of past A key indicator of past landslippagelandslippage is is anomalous topographic anomalous topographic expression, which expression, which becomes increasingly becomes increasingly mollified with age, as mollified with age, as shown in profiles at left shown in profiles at left

Landslides tend to Landslides tend to form coalescing form coalescing complexes, with complexes, with hummocky hummocky topography and topography and deranged drainage deranged drainage patterns. Toe patterns. Toe undercutting or undercutting or tectonic uplift are tectonic uplift are common triggerscommon triggers

TOPOGRAPHIC TOPOGRAPHIC EXPRESSION OF EXPRESSION OF

LANDSLIDESLANDSLIDES

RECOGNIZING OLD RECOGNIZING OLD LANDSLIDESLANDSLIDES

Recent landslides like those at left are easily noticed, but older complexes can be difficult to discern, especially under thick tree cover. The hummocky topography in the right image is typical oflandslide-prone terrain.

INCIPIENT INCIPIENT LANDSLIPPAGELANDSLIPPAGE

Incipient landslides Incipient landslides are those that are are those that are beginning to fail, but beginning to fail, but have not fully have not fully rupturedrupturedThey are easily They are easily identified by their identified by their arcuatearcuate tensile tensile scarps crossing scarps crossing otherwise intact otherwise intact slopes slopes

OLD BEDROCK OLD BEDROCK LANDSLIDESLANDSLIDES

Old bedrock landslides can be very difficult to identify. Key indicators are anomalous isolated benches and converging parallel drainages

HEADSCARPSHEADSCARPS

The arcuate nature of a landslide headscarp is due to tensile pull-apart as the slide mass translates downslope. It is easy to estimate the depth of sliding from the opposing scarps

BUCKET AUGERSBUCKET AUGERSBedrock landslides are best explored using large diameter bucket auger borings, which can be downhole logged

St. Francis Dam was unknowingly constructed against a paleolandslide developed in pre Cambrian age Pelona Schist in 1924-26.

535 million m3 of schist in the dam’s left abutment detached and slid downslope, destroying the dam. Relaxation movements of up to 3 m were noted at elevations more than 60 m above the left abutment

SAN GABRIEL DAMSAN GABRIEL DAMThe San Gabriel Dam at The San Gabriel Dam at the Forks Site would the Forks Site would have been the largest have been the largest dam in the world when dam in the world when construction began in construction began in September 1928September 1928These views show right These views show right abutment excavations at abutment excavations at the Forks site in June the Forks site in June 19291929Plans called for 200,000 Plans called for 200,000 ydsyds33 excavation on each excavation on each abutment abutment

Three views showing before (left) and during (middle) and after (right) the massive detonation of the dam’s right abutment. The contractor detonated 87,430 kg of dynamite. On September 16, 1929 a massive slide of the right abutment occurred, bringing down 153,000 m of additional rock. This led to an investigation and eventual cancellation of the project by the State of California.

LANDSLIDE DAMSLANDSLIDE DAMS

Landslide dams perturb channel profiles. Lessened gradient upstream of blockage and increased gradients through the obstruction, as shown above. Channels often swing around the obstructions in a convex outward pattern.

CHANNEL PROFILE

Example of a channel profile perturbed by landslide Example of a channel profile perturbed by landslide damming in Fishtail Creek, Grand Canyon. Note lower damming in Fishtail Creek, Grand Canyon. Note lower gradient upstream of obstruction and increased gradient upstream of obstruction and increased gradient on downstream side. gradient on downstream side.

CHANNEL SCOURCHANNEL SCOUREngineering geologists Engineering geologists are often asked to are often asked to estimate depth of estimate depth of channel scour, channel scour, especially for bridgesespecially for bridgesThese are during and These are during and after views of a desert after views of a desert flash flood in June 1969 flash flood in June 1969 which developed 8 feet which developed 8 feet high standing waves. high standing waves. Note children for scale Note children for scale in lower image, taken a in lower image, taken a few weeks later. few weeks later.

Arroyo Arroyo PasajeroPasajeroWash OutWash Out

A flash flood on March A flash flood on March 10, 1995 brought a peak 10, 1995 brought a peak flow of 33,000 flow of 33,000 cfscfsthrough a constricted through a constricted overcrossingovercrossing for for Interstate 5 near Interstate 5 near Coalinga, CA. The Coalinga, CA. The channel was channel was downcutdowncutby almost 30 feet, by almost 30 feet, undermining the bridge undermining the bridge caissons and dropping caissons and dropping the twin spans. 7 the twin spans. 7 people were killed. people were killed.

DAMS THAT WOULD DAMS THAT WOULD NOT HOLD WATERNOT HOLD WATER

Escondido (upper) and Morena (lower) Dams were both constructed around 1900 in southern California. They lost large volumes of water through alluvial gravels beneath the embankments.

Escondido Dam never filled beyond 2/3 capacity, losing 100,000 gpd. Morena lost between 33,500 to 58,000 gpd, depending on head.

KARST KARST FOUNDATIONSFOUNDATIONS

Cross section through the Cross section through the foundation for Kentucky Dam on foundation for Kentucky Dam on the Tennessee River (TVA, 1949).the Tennessee River (TVA, 1949).The Pleistocene Valley floor was The Pleistocene Valley floor was about 70 feet lower than at about 70 feet lower than at present, with deep solution present, with deep solution cavities extending 200 feet cavities extending 200 feet beneath the channelbeneath the channel734,000 sacks of cement were 734,000 sacks of cement were pumped into the foundation pumped into the foundation during constructionduring construction (20,000 m(20,000 m33))

Anchor Dam was built by the U.S. Bureau of Anchor Dam was built by the U.S. Bureau of Reclamation 56 km west of Thermopolis, WY in 1957Reclamation 56 km west of Thermopolis, WY in 1957--60. 60. Reservoir water seeps downward into Reservoir water seeps downward into karstifiedkarstified redbedsredbedsof the Permian age Goose Egg and Triassic age of the Permian age Goose Egg and Triassic age ChugwaterChugwater formations. The dam has never retained any formations. The dam has never retained any significant volume of water.significant volume of water.

OUTOUT--OFOF--PLANE DISCONTINUITIESPLANE DISCONTINUITIES

ValleyValley--side stress relief joints tend to form parallel to side stress relief joints tend to form parallel to free cliff faces, in response to stress changes free cliff faces, in response to stress changes engendered by excavation of the valleys, slope creep engendered by excavation of the valleys, slope creep and thermallyand thermally--induced stresses. They are dangerous induced stresses. They are dangerous because they cannot be seen in casual mapping. because they cannot be seen in casual mapping.

Exfoliation joints exposed in glacial cirque of Little Shuteye PExfoliation joints exposed in glacial cirque of Little Shuteye Pass,ass, in in California’s Sierra Nevada Mountains, in the Mt. Givens California’s Sierra Nevada Mountains, in the Mt. Givens granodioritegranodiorite

SHEET JOINTSSHEET JOINTSSheet joints, valley side Sheet joints, valley side joints or exfoliation joints joints or exfoliation joints all describe the same all describe the same featuresfeaturesWhen the valley side is When the valley side is excavated new joints excavated new joints form beneath the form beneath the unloadingunloadingThis caused problems at This caused problems at Mammoth Pool dam site Mammoth Pool dam site in California, shown herein California, shown hereTerzaghiTerzaghi recommended recommended placing fill for dam and placing fill for dam and grouting the joints as grouting the joints as construction progressed construction progressed

VALLEYVALLEY--SIDE JOINT FAILURE MODESSIDE JOINT FAILURE MODES

ValleyValley--side joints are particularly treacherous because they are side joints are particularly treacherous because they are usually inclined at close to 45usually inclined at close to 45--ØØ/2 degrees from vertical (around 60 /2 degrees from vertical (around 60 degrees), which offers the least shear resistance to slippage.degrees), which offers the least shear resistance to slippage.

Catastrophic Catastrophic rockfallrockfall typical of steeply inclined bluffs typical of steeply inclined bluffs with outwith out--ofof--plane discontinuities. This example is from plane discontinuities. This example is from 6 Mile Wash in Marble Canyon, Arizona6 Mile Wash in Marble Canyon, Arizona

HAUNCHES OF GLEN CANYON DAMHAUNCHES OF GLEN CANYON DAM

Glen Canyon Glen Canyon damsitedamsitehas prominent has prominent haunches formed by haunches formed by curvalinearcurvalinear valley valley side joints. The side joints. The upper vertical walls upper vertical walls are controlled by are controlled by regional systematic regional systematic joints.joints.

GLEN CANYON TUNNEL FAILUREGLEN CANYON TUNNEL FAILURE

The Glen Canyon Powerplant Tunnel had a series of deadly block failures during excavation because of wedges formed between inclined valley side and systematic joints, sketched above

BLOCK FAILURESBLOCK FAILURES

Valley side joints are inclined. Where these intercept near vertical systematic joints, massive blocks are formed on steep inclines. When these are undercut the blocks slide off. A little bit of water hastens the process by decreasing the friction markedly.

FontenelleFontenelle DamDam came perilously close to failing came perilously close to failing catastrophically in Sept 1965 during its initial filling. catastrophically in Sept 1965 during its initial filling. Seepage emanated from the right abutment. The same Seepage emanated from the right abutment. The same failure mode befell Teton Dam 11 years later.failure mode befell Teton Dam 11 years later.

Seepage Seepage percolated percolated along along undiscovered undiscovered valleyvalley--side side joints (#7) in joints (#7) in bedded bedded sedimentary sedimentary rocks at rocks at position #8position #88 lines of grout 8 lines of grout holes were holes were then drilled and then drilled and filled with filled with 203,500 sacks 203,500 sacks of cement of cement

FONTENELLE DAM

The ZionThe Zion--Mt Carmel Tunnel Gallery 3 failure occurred in April 18, Mt Carmel Tunnel Gallery 3 failure occurred in April 18, 1958, spilling 84,000 tons of Navajo Sandstone onto the slope 1958, spilling 84,000 tons of Navajo Sandstone onto the slope below and damaging the tunnelbelow and damaging the tunnel

Cross section of Pine Canyon at location of Cross section of Pine Canyon at location of the Gallery 3 the Gallery 3 rockfallrockfall. Note valley. Note valley--side joints side joints

Unravel geomorphic progressionUnravel geomorphic progression –– Cliffs tend Cliffs tend to retreat in episodic steps. All of these steps to retreat in episodic steps. All of these steps are usually discernable at any given site.are usually discernable at any given site.

Stage 1 is gross cliff retreat immediately following a major detStage 1 is gross cliff retreat immediately following a major detachment. achment. After the remaining overhang drops, a completely smooth face forAfter the remaining overhang drops, a completely smooth face forms, ms, arbitrarily designated here as Stage 2.arbitrarily designated here as Stage 2.

Stage 3 a sloping cliff toe forms, controlled by valleyStage 3 a sloping cliff toe forms, controlled by valley--side joints. side joints. In Stage 4 crown blocks have detached along inclined valleyIn Stage 4 crown blocks have detached along inclined valley--side side jointsjoints

Stage 5 is overhanging situation preceding massive Stage 5 is overhanging situation preceding massive detachment of detachment of block(sblock(s) along inclined valley) along inclined valley--sidesidejointsjoints

CONCLUSIONSCONCLUSIONSEngineering geologists need to unravel Engineering geologists need to unravel the physical agents responsible for the physical agents responsible for shaping the landscape around usshaping the landscape around usThis requires through background This requires through background research, patient site mapping, a focused research, patient site mapping, a focused program of subsurface exploration, and a program of subsurface exploration, and a critical analysis of landformscritical analysis of landformsWe will not identify those geologic We will not identify those geologic features we are not specifically looking features we are not specifically looking for, or with which we have little previous for, or with which we have little previous exposure or experience exposure or experience

WEB POSTINGWEB POSTING

If you would like to use any part of this If you would like to use any part of this presentation, you are welcome to presentation, you are welcome to download it without written permissiondownload it without written permissionIt will be posted on my website at:It will be posted on my website at:

www.umr.edu/~rogersdawww.umr.edu/~rogersdaAn accompanying article is included in An accompanying article is included in the symposium proceedings on CDthe symposium proceedings on CD--ROMROM